Over the last several decades, vehicle stability systems have increased both their sophistication and market penetration. Early vehicle stability systems sensed the speeds of the four wheels of the vehicle and continuously monitored these speeds to detect a speed difference which was interpreted as wheel slip. Based upon a sensed wheel speed difference, the delivery of torque to the vehicle wheels would be adjusted to reduce or eliminate such wheel speed differences and slip. Early systems energized a clutch in a transfer case resulting in a shift of delivery of 100% of the torque to the rear wheels to 50% to the front wheels and 50% to the rear wheels. Such a system is disclosed in co-owned U.S. Pat. No. 4,989,686 to Miller et al.
Later, more sophisticated control systems utilized timed, stepwise or incremental actuation of the transfer case clutch. Such a system is disclosed in co-owned U.S. Pat. No. 5,407,024 to Watson et al.
Inherent in the control architecture of these and numerous other vehicle traction and stability control systems is the fact that the systems do not commence operation and torque redistribution until a reduced traction and slip event has occurred and been detected. Thus, there may be a brief period of time between wheel slip and recovered stability that may be perceptible by the vehicle operator and passengers. While it is true that the slip threshold may be reduced to any quantative value, from a practical standpoint, the slip threshold cannot be reduced without limit as a small threshold value will result in nuisance engagements of the stability system which may be more noticeable to the vehicle occupants than the less frequently encountered brief interval between slip and correction during an event for which the system was intended.
The foregoing suggests that a system which monitors various vehicle parameters and proportions drive torque to the four vehicle wheels to provide vehicle stability would be desirable.